Fluorescent lamp ballasts have been typically designed in the U.S. with input current power factor greater than 0.9. (Power factor is defined as the ratio of real power delivered to a circuit to the product of the rms current and voltage at its input.) The recent trend is to exceed power factors of 0.9 and even 0.95. Unfortunately, the requirement for simple, low-cost power factor correction circuits generally conflicts with the requirement for a lamp current crest factor of less than 1.7. (Lamp current crest factor is the ratio of peak lamp current to its rms current.) In addition, the efficacy of a fluorescent lamp decreases as lamp current increases. And, if a modulated current is supplied to a lamp, overall efficacy is lower than if just the average value of the current is supplied in an unmodulated fashion.
An integrated boost circuit that meets the power factor and lamp current crest factor requirements described hereinabove is presented in U.S. Pat. No. 5,408,403 of L. R. Nerone and D. J. Kachmarik, filed Aug. 25, 1992 and assigned to the instant assignee. The integrated boost circuit is used for powering a load with bi-directional current and comprises a full-wave rectifier, a series half-bridge converter, and a boost converter. The series half-bridge converter includes a first switch interposed between the bus conductor and a bridge-switch end of the load circuit; a second switch interposed between a ground conductor and the bridge-switch end of the load circuit; and a switching control circuit for alternately switching on the first and second switches. The boost converter comprises a boost capacitor connected between the bus and ground conductors, the level of charge on the boost capacitor determining the bus voltage on the bus conductor; a boost inductor connected by a one-way valve to the boost capacitor for discharging its energy into the boost capacitor; and a low-impedance path for periodically connecting a load end of the boost inductor to the ground conductor, thereby charging the boost inductor.
Unfortunately, circuits such as the integrated boost circuit are impractical for low-cost applications such as compact fluorescent lamps with integral ballasts.
Alternatively, simple circuits, such as, for example, a family of circuits known as valley fill circuits can be used for fluorescent lamp ballasts, but at the expense of low input current power factor or high lamp current crest factor. In a valley fill circuit, an electrolytic capacitor is charged to half the peak line voltage. The electrolytic capacitor is connected to the ballast dc bus via a diode. As long as the line voltage remains above the capacitor voltage, the line supplies the ballast. When the line voltage goes below the capacitor voltage, the diode conducts, thereby feeding the ballast with current and reverse-biasing the rectifier bridge diodes. Thus the voltage supplied to the ballast follows the line voltage from an angle of 60.degree. to an angle of 120.degree. and follows the capacitor voltage near the zero-crossings of the line(referred to as "valleys").
In a typical valley fill circuit, the conduction angle of the line current allows the circuit to have a power factor that is just slightly above 0.9. At the same time, the modulated bus voltage results in a lamp current crest factor that is just slightly below 1.7. The margins by which the circuit meets the power factor and lamp current requirements are very narrow, such that the normal spread of component values in production could cause a substantial fraction of the ballasts to fail to meet them. Therefore, a simple, low-cost, high-performance high power factor correction circuit is still needed.